Special Senses

• Special Senses


  • Smell (olfaction)
  • Taste (gustation)
  • Hearing (audition)
  • Vision
  • Balance (equilibrium)
• Touch is not a special sense but a general sense
• General Senses


  • Touch
  • Pain
  • Temperature
  • Taste
  • Light
  • Sound
  • Head movement
  • Smell
• Special senses


  • Convey specific information due to specialised receptors
• Specialised receptors


  • Mechanoreceptors (respond to stretch, bend or deformation)
  • Photoreceptors (contain molecules that respond to light)
  • Chemoreceptors (respond to chemicals)
• These receptors convert the stimulus into action potentials that travels to the brain for interpretation
• Receptors for smell and taste are chemoreceptors
• Chemoreceptors respond to chemicals in a fluid
• Odorants


  Volatile (gaseous) chemicals that must dissolve in mucus
• Olfaction (smell)


  1. Odorants reach olfactory epithelium and dissolve in mucus

  2. Odorant binds and opens gated channels

  3. Action potential generated
• Olfactory sensory neurons


  • Located in olfactory epithelium
  • Have cilia surrounded by mucus

  
• Olfactory pathway


  1. Olfactory sensory neurons form olfactory nerve (CN I)

  2. Action potentials travel to olfactory cortex of temporal lobe

  3. Information takes two pathways: to frontal lobe for conscious interpretation and to hypothalamus/limbic system for emotional responses
• Smell can signal danger, trigger fight or flight response, stimulate salivation and digestive system
• Gustation (taste)


  Provided by gustatory epithelial cells in taste buds
• Gustatory epithelial cells


  • Have microvilli called gustatory hairs
  • Food chemical must be dissolved in saliva
  • Food chemical binds to receptor of hair cells
  • Action potential generated
• Gustatory pathway


  Facial (VII), Glossopharyngeal (IX) and Vagus (X) nerves carry action potentials from taste buds to gustatory cortex of insula and hypothalamus/limbic system
• Taste can trigger reflexes involved in digestion, increase secretion of saliva and gastric juice, and initiate protective reactions
• Much of what we "taste" is actually smelled
• Major areas of the ear

  • External (outer) ear
  • Middle ear (tympanic cavity)
  • Internal (inner) ear

  
• External ear


  • Auricle (pinna) funnels sound waves into external acoustic meatus
  • External acoustic meatus transmits sound waves to tympanic membrane

  
• Middle ear


  • Air-filled cavity in temporal bone
  • Contains auditory ossicles (malleus, incus, stapes) that transmit and amplify vibrations from tympanic membrane to oval window
  • Contains pharyngotympanic tube that equalises air pressure

  
• Internal ear


  • Bony labyrinth (vestibule, semicircular canals, cochlea) filled with perilymph
  • Membranous labyrinth (interconnecting sacs and ducts) filled with endolymph
• Cochlea


  • Spiral, bony chamber that extends from vestibule
  • Contains cochlear duct with spiral organ (of Corti)
• Spiral organ (of Corti)


  • Organ for hearing
  • Composed of cochlear hair cells sandwiched between basilar membrane and tectorial membrane
  • Pressure waves in perilymph cause movement of basilar membrane, bending hair cells to generate action potentials
• Pathway of sound waves through the ear

  1. Sound waves vibrate tympanic membrane

  2. Auditory ossicles vibrate oval window, amplifying pressure

  3. Pressure waves in perilymph vibrate basilar membrane and bend hair cells

  4. Action potentials generated in hair cells travel via vestibulocochlear nerve

  
• Oval window: opening that transmits pressure waves to cochlea

  Round window: membrane that absorbs pressure waves
• Auditory pathway to the brain

  Action potentials travel from hair cells via cochlear branch of vestibulocochlear nerve to medulla oblongata, midbrain auditory reflex center, then to auditory cortex in temporal lobe

  
• Balance or equilibrium


  Response felt by head movements, providing information about head position and movement
• Types of equilibrium


  • Static equilibrium (evaluating head position relative to gravity)
  • Dynamic equilibrium (evaluating head rotation and angular movement)
• Vestibule


  • Contains saccule and utricle with hair cells embedded in jellylike substance with otoliths (ear stones)
• Static equilibrium


  Gravity pulls on otoliths, distorting jelly and bending hair cells to stimulate depolarisation
• Static equilibrium


  Evaluating the position of the head relative to gravity (up/down) and forward/backward. Information supplied when the head is motionless, or moving in a straight line – detected by hair cells in the vestibule (saccule and utricle)
• Dynamic equilibrium


  Evaluates the position of the head while it is rotating, or moving in an angular direction – detected by hair cells in the semicircular canals
• Vestibule


  • Contains 2 membranous sacs
  • Saccule: continuous with cochlear duct
  • Utricle: continuous with semicircular canals
  • Both contain equilibrium receptors
  • Utricle & saccule contain hair cells embedded in jellylike substance
  • Tiny ear stones (otoliths) are embedded in the surface of the jelly
• Static equilibrium


  1. When head position is changed, gravity pulls on the stones and distorts the jelly, bending the equilibrium hair cells in vestibule

  2. Distortion of the hair cells stimulates depolarisation
• Semicircular canals


  • 3 fluid filled canals that lie in three planes of space
  • Equilibrium hair cells respond to rotational movement of the head
• Dynamic equilibrium: Semicircular canals

  1. Contain hair cells surrounded by endolymph

  2. Angular movement moves endolymph

  3. Endolymph movement bends and distorts hair cells
• Equilibrium pathway


  1. Distortion of the hair cells (vestibule and semicircular canals) stimulates depolarisation of vestibular division of the vestibulocochlear nerve

  2. Action potentials travel to: the reflex centres (vestibular nuclei) of the brainstem, cerebellum, vestibular cortex of the insula
• Movement of the eyeball

  • Six (6) extrinsic eye muscles allow movement of each eyeball
  • Innervated by cranial nerves III, IV and VI
  • Enable the eyes to be fixed on stationary or moving objects and keep both eyes fixed on the same object at the same time
• Structures of the eyeball

  • Cornea: transparent layer that covers the anterior structures of the eye
  • Iris: coloured portion of the eye, contains muscles that can constrict and dilate to change the diameter of the pupil, controlled by sympathetic and parasympathetic division of nervous system
  • Pupil: round central opening that allows light to enter the eye